<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <button onclick="sum()">加</button>
    <button onclick="sub(a,b)">减</button>
    <button onclick="mul(a,b)">乘</button>
    <button onclick="div(a,b)">除</button>
    <script>
        var a = window.prompt();
        var b = window.prompt();
    
       
        function sub(a,b){
            a-b;
        }
        function mul(a,b){
            a*b;
        }
        function div(a,b){
            a/b;
        }
    
    </script>

</body>
</html>